Two-stroke (alternatively referred to as two-cycle) engines have been known for many years and have been applied in a range of applications. One class of two-stroke engines is the class of engines operating on a normally gaseous hydrocarbon, most commonly natural gas, under lean burn conditions. Such engines are generally large, slow running engines of a stationary design and find application in the driving of rotating and reciprocating equipment, such as compressors and electric generators. One example of commercially available engines is the Ajax® series of engines manufactured and sold by the Cooper Energy Services division of Cooper Cameron Corporation. The Ajax engines are two-stroke engines having from one to four cylinders. When used to drive a compressor, the Ajax engines are commonly employed in a configuration in which the cylinders of a reciprocating compressor are driven from the same crankshaft as the cylinders of the engine.
Engines of this class generally operate at low speeds, that is speeds of the order of from several hundred to a thousand revolutions per minute. The engines are generally operated in a constant speed mode, in which a substantially constant speed is maintained under a variety of engine loads. As the power demand placed on the engine is increased, the combustion efficiency and performance of the engine improves.
Recent environmental regulations have been increasing the emphasis on the importance of reducing the levels of partially burned fuel constituents from the exhaust of stationary engines. These regulated exhaust emissions consist of CO, NMHC, and formaldehyde (CH2O). An oxidizing catalyst in the exhaust stream will produce dramatic reductions in the levels of these emissions. Accordingly, there is a need for a way to reduce carbon monoxide, formaldehyde and volatile organic compounds (VOC) emissions from engines in this class.
One method of reducing the amount of such emissions in other types of internal combustion engines is to employ a catalytic converter in the exhaust system of the engine. The catalytic converter converts such emissions in the exhaust gases to less harmful emissions before they are emitted to the atmosphere. However, that has proven more difficult in practice. Previous industry experience with applying oxidizing converters to 4-stroke natural gas fueled engines indicates satisfactory results relative to the removal efficiencies of the subject emissions and the duration of operating time accumulated between catalyst cleaning and/or element replacement. However, previous tests of oxidizing catalysts with 2-stroke natural gas fueled engines have demonstrated good removal efficiencies for only short time periods. Therefore, currently available lean burn catalyst systems are limited to 4-stroke engine applications.
The majority of oxidation catalysts use a combination of platinum (Pt), rhodium (Rh), and palladium (Pd). Under the lean conditions that these engines are run, there is excess oxygen present in the exhaust. With excess oxygen present, oxidation catalysts are effective at eliminating carbon monoxide, formaldehyde and VOC emissions.
All of the chemical reactions that occur in a catalyst occur at the surface. So, any decrease in the surface area or the number of active sites available of the catalyst results in a decrease in the effectiveness of the catalyst. The specific deactivation mechanisms present in 2-stroke lean burn natural gas engines include selective poisoning and non-selective poisoning.
Selective poisoning occurs when a material reacts directly with the catalytic material rendering it unable to function as a catalyst. Poisoning is generally a reversible process, which is treated by using heat, washing or simply removing the poison from the exhaust stream. Sulfur from engine oil in the exhaust stream is a major contributor to catalyst poisoning.
Non-selective poisoning is also referred to as masking or fouling. It is the result of materials in the exhaust flow that accumulate on the catalyst surface. Phosphorous compounds and other materials, which are common in lubricating oils and in partially burned combustion products, can be found on the surface of the catalyst.
Differences in catalyst performance are also affected by temperature. Higher temperatures increase catalyst efficiency and may impede poisoning. The difference in temperatures is why 4-stroke natural gas fueled engines have been successfully outfitted with catalytic converters and why there is still a need for them in 2-stroke natural gas fueled engines. The difference in temperatures is due to the differences in engine design. Because of the scavenging process, 2-stroke engines have cooler exhaust temperatures than 4-stroke engines that consequently hinder exhaust performance.
M. DeFoort et al. of Colorado State University reported these problems and differences at the Gas Machinery Conference 2002 in Nashville, Tenn. on Oct. 8, 2002, in their paper entitled Performance Evaluation of Oxidation Catalysts for Natural Gas Reciprocating Engines. This paper discloses the use of a catalyst in an attempt to treat the exhausts from 2-stroke and 4-stroke lean burn natural gas fueled engines. The catalyst efficiency dropped from 95% to 80% for CO and from 75% to 45% for formaldehyde during the catalyst aging process for a large bore 2-stroke engine (about 200 hours). However, the results for the medium bore 4-stroke engine were better due to the nearly 200 degree F. higher catalyst temperatures. The catalyst efficiency dropped from 99.2% to 97.7% for CO and from essentially 100% to 67% for formaldehyde during the catalyst aging process (about 150 hours).
The specific 2-stroke engine used was a Cooper-Bessemer GMV-4TF stationary internal combustion engine having four cylinders with a manufacturer's sea level rating of 440 brake-horsepower (bhp) at 300 rpm. The cylinders were 14 inches in diameter with a 14-inch stroke. Air was delivered to the engine using a supercharged air delivery system. During the scavenging process, about half of the air supplied to the engine passed through the engine and was not trapped in the cylinder. The other half of the supplied air was trapped in the cylinder and participated in the combustion process. The catalyst was contained in a housing having four units, each measuring 12″×16″×3″. The housing was inserted in the exhaust line, but its location is not clear from the article since FIG. 6.1 showing its location was not published with the article.
M. DeFoort et al. analyzed the catalyst used with the 2-stroke engine. They found that the leading edge of the catalyst had three oxides not present in the trailing edge of the catalyst. These were oxides formed from copper (CuO), phosphorus (P2O5) and zinc (ZnO). Sulfur also played a role in the deterioration of the catalyst. The elements copper, phosphorus and zinc, plus other elements such as iron and calcium, contributed to the deactivation of the catalyst, all of which are known catalyst poisons originating from engine lubricants and coolants. In addition, black soot was found on the leading edge of the catalyst.
In summary, M. DeFoort et al. concluded based on their results that oxidation catalysts were not likely to be effective for large bore 2-stroke lean burn engines. The oxidation catalyst showed clear signs of poisoning in a relatively short period of time (less than 250 hours) when compared to the expected lifespan of the catalyst.
While catalytic converters for a 2-stroke engine are known in the art, their application has been limited to 2-stroke engines of much smaller capacity and operating at speeds far greater than those of the class of engines addressed by the present invention. See, for example, catalytic converters disclosed in U.S. Pat. No. 6,277,784 (for small engines); and muffler/catalytic converter combinations disclosed in U.S. Pat. No. 4,867,270 (for portable hand tools); U.S. Pat. No. 5,866,859 (for portable work tools); U.S. Pat. No. 5,916,128 (for small 2-stroke engine); U.S. Pat. No. 6,109,026 (for portable work tools); U.S. Pat. No. 6,315,076 (for small engines); U.S. Pat. No. 6,403,039 (for small engines); and U.S. Pat. No. 6,622,482 (for small engine applications).
To date because of the problems noted by M. DeFoort et al., such catalytic converter exhaust systems have not been applied to large capacity 2-stroke lean burn engines operating on a normally gaseous hydrocarbon fuel and operating at speeds at or below about 1000 rpm.
Accordingly, there is a need for a solution to the problem of achieving lower carbon monoxide and formaldehyde emissions in the exhaust from large capacity 2-stroke lean burn engines operating on a normally gaseous hydrocarbon fuel and operating at speeds at or below about 1000 rpm, while maintaining a satisfactory level of catalyst efficiency and requiring little maintenance over and above the existing maintenance schedules.